1. Field of the Invention
The present invention relates to a skeleton type BLDC motor, and more particularly, to a BLDC motor which reduces resonance noise by reducing cogging torque.
2. Description of the Background Art
FIG. 1 is a perspective view of a skeleton type BLDC motor in accordance with the conventional art, FIG. 2 is a cross-sectional view of a skeleton type BLDC motor in accordance with the conventional art; and FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2.
The conventional BLDC motor is made up of a stator 102 to which the power is applied; a rotor 104 disposed at an inner circumferential surface of the stator 102 with an air gap and rotated by interlocking with the stator 102; and a rotary shaft 106 mounted at the center of the rotor 104 and rotated together with the rotor 104, for transferring a rotary force to the outside.
A PCB 108 having a driver circuit for driving the rotor 104 therein is installed at one side of the stator 102, and the PCB 108 is embedded in a PBC cover 110.
The stator 102 is made up of a stator core 116 in which a plurality of sheets are laminated and coupled by a rivet, and in which a first pole shoe 112 and a second pole shoe 114 which accommodate the rotor 104 are formed as one body; a pair of bobbins 118 and 120 fixed to the stator core 116; and a pair of coils 122 and 124 wound on the pair of bobbins 118 and 120, respectively, the pair of coils to which the power is alternately applied.
For the initial starting, detent grooves 126 disposed symmetrically to the rotary shaft line of the rotor 104 are respectively formed at the first pole shoe 112 and the second pole shoe 114 in order that the rotor is not aligned at a position where torque of the rotor is zero.
As shown in FIG. 4, the rotor 104 includes: a circular magnet 130 whose one side is a north pole and the other side is a south pole on the basis of a magnetic pole separating line formed radially from the center of the rotor 104; and a magnet frame 132 disposed at an inner circumferential surface of the magnet 130 and having a rotary shaft 106 fixed to its center.
The rotor 104 is accommodated inside a motor housing 140, and a pair of bearings 142 and 144 rotatably supporting the rotary shaft 106 are disposed at the motor housing 140.
A location detecting sensor (not shown) for detecting a rotary location of the rotor 104 is disposed at the PCB cover 110, and the location detecting sensor is received in a sensor receiving portion 136.
An operation of the BLDC motor in accordance with the conventional art having such a construction will be described. When the power is applied, the location detecting sensor detects a rotary location of the rotor 104 by using information of the magnetic pole part line 128 of the rotor 104, and the DC power is applied to one of the pair of coils 122 and 124 on the basis of the detection result. Then, the stator core 116 is excitated to rotate the magnet 130. Then, the magnet frame 132 mounted on the magnet 130 is also rotated, and the rotary shaft 106 fixed to the magnet frame 132 is rotated.
At this time, the rotor 140 is not aligned to a position where torque is zero by means of the detent grooves 126 formed to have a large interval relatively to the first pole shoe 112 and the second pole shoe 114.
However, in the conventional skeleton type BLDC motor having such a construction, cogging torque, which is a tangential force whose tendency is to move in equilibrium with the minimum magnetic energy, is inevitably generated. Such cogging torque is generated between the first pole shoe and the second pole shoe regardless of the current.
In addition, when the magnetic pole separating line of the rotor passes the detent grooves by the rotation of the rotor, an air gap between the rotor and the stator changes and therefore magnetic flux density changes. Hereupon, higher degree of cogging torque as well as basic degree of cogging torque is generated. The cogging torque of higher degree becomes the main cause of resonance noise of the motor, thereby increasing noise of the motor.
In order to reduce the generation of the cogging torque, though research and development in noise reduction has been a primary focus by using a method for improving a magnetizing method of the magnet or a method for reducing the vibration source by changing a shape of the magnet, there is a problem that basic degree cogging torque as well as higher degree (the fourth degree, fifth degree, . . . ) cogging torque is reduced. Thus, there is a problem that the performance of a motor which requires a certain level of cogging torque is deteriorated.